Daisy, your Osteospermum in the photo isn't hardy. It's not likely to last much longer this year. They're treated as annuals in this country, unless it's the one hardy one which is Osteospermum jucundum.

Some people take cuttings from their Osteos if they're particularly pretty ones.

this one is Osteospermum : Spider Purple

[ photo link : https://picasaweb.google.com/109078447128567897634/Osteospermum?authkey=Gv1sRgCIfdyJLdqYqzpwE ]

It has had 2 flowering cycles, in April when I bought the plant and in June. So is it done for the season?

Is it too early or late to take cuttings from this plant?

and also I found a mushroom growing near the root crown. Is it a symptom of root fungus/root disease ?
What is the remedy?

Thanks.

You can take cuttings now. As you live in CA the plant will last until spring then start looking weak - you can take cuttings at any time.